Flexible fuel sensor system

ABSTRACT

A method and apparatus for determining the percentage of two different fluids having substantially different dielectric properties such as a gasoline/alcohol or a water/oil mixture. An inductive coil element immersed in the mixture exhibits a distributed capacitance level which is directly proportional to the dielectric constant of the fluid mixture surrounding the coil which, in turn, is directly proportional to the percentage of each fluid in the mixture. The loops of wire in the coil act as equivalent electrodes in creating a capacitance between each loop. The dielectric constant of a mixture will increase with the percentage of the higher dielectric fluid thus increasing the capacitance. An amplifier is constructed that oscillates at the resonant frequency of the LC circuit formed by the coil in the mixture. The resonant frequency decreases as capacitance increases. The subject invention includes a circuit which converts a frequency signal generated by energization of the coil to a voltage signal which, in turn, is sent to an engine controller. The output signal is used by the engine controller to selectively adjust the fuel ratio and the spark timing of the engine in accordance with the percentage of alcohol in the fuel mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for determining thepercentage of two known liquids in a mixture where the two known liquidspossess substantially different dielectric properties. The combinedpercentage of each of the two known liquids in a mixture is directlyproportional to the distributed capacitance of an insulated wire coilimmersed in the mixture.

2. Description of the Related Art

In the automotive industry there are demands for an automobile that canperform on both gasoline and alcohol. However, typical internalcombustion engines must have selectively adjustable parameters, such asspark timing, for efficient combustion when running on the differentfuels. One design that attempts to meet this demand is an opticalrefraction index sensor which utilizes the relationship between thepercentage of alcohol in a fuel mixture and the angle of lightrefraction through the fuel mixture.

SUMMARY OF THE INVENTION

Due to shortages of oil and the promulgation of stringent emissionstandards, the automotive industry has been under intense pressure todevelop internal combustion engines that can perform efficiently usingalternative fuels such as methanol and ethanol. Currently, only methanoland ethanol are viable fuel alternatives to gasoline since both are ableto create a similar amount of power in spark ignited engines.

In order to accommodate the fluctuating supply of alcohol based fuels,vehicle manufacturers have to modify their gasoline fuel systems toaccept alcohol based fuels or gasoline/alcohol fuel mixtures. Thesubject invention is a method and apparatus for detecting the percentageof alcohol in a gasoline/alcohol mixture and relaying that informationto an engine controller or microprocessor or other type of controller sothat the spark timing and fuel ratio can be adjusted accordingly.Analogously, the subject invention also has the ability to detect thepercentage of two known liquids (each of which may be a mixture itself)within a mixture wherein the liquids have substantially differentdielectric properties.

The fuel mixture detection device of the subject invention consists of acoil which is immersed in the fuel mixture. A distributed capacitance isgenerated between the windings of the coil upon energization of thecoil. The wire turns act as equivalent electrodes of a capacitor and thefuel mixture acts as a dielectric medium. This distributed capacitanceof the coil is directly proportional to the dielectric constant of themixture which, in turn, is directly proportional to the percentage ofeach fuel in the mixture. This assumes that the sensor is dealing withknown fluids as a base (each of which could be a mixture of otherfluids) so long as the two known fluids have significantly differentdielectric constants. This distributed capacitance 21 is approximatelyrepresented by the equivalent circuit designated as a dashed linecapacitor symbol and labeled C_(D) as shown in FIG. 1 and in FIG. 3A.

The coil may be supported within a hollow casing defining a chamber.Interconnecting tubing is used to channel the liquid mixture to the coilwithin the chamber. An electrical circuit is interfaced with the coiland mounted immediately adjacent. In this manner, only the coil and theinterconnections to the circuit are in contact with the liquid mixture.Without this type of arrangement, long interconnection cables would beneeded which would undesirably add stray or parasitic impedance to thecircuit thereby dwarfing the capacitance of the coil, rendering it aless accurate indicator of the liquid mixture flowing between theelectrodes.

The circuit of the subject invention includes a unique oscillatorutilizing the submerged coil. A relationship exists between the resonantfrequency and the capacitance of the coil whereby the resonant frequencydecreases as the capacitance of the coil increases. The dielectricconstant of the mixture is determined by using the coil as the resonantelement in the oscillator, thereby generating an oscillation frequencywhich is inverse proportionally related to the dielectric constant ofthe liquid mixture. The circuit then provides a means whereby thefrequency is converted to a voltage output and sent to a controller forprocessing. In the case of a alcohol/gasoline fuel mixture for internalcombustion engines, the output signal of the sensor is sent to an enginecontroller or computer means.

It is an object of the present invention to provide a method andapparatus for determining the percentage of two known liquids in amixture where the two known liquids possess significantly differentdielectric constants.

It is another object of the present invention to determine thepercentage of alcohol in a gasoline/alcohol fuel mixture for use inselectively adjusting engine parameters.

It is yet another object of the present invention to utilize therelationship between the distributed capacitance of a coil and itsresonant frequency to determine the percentage of two liquids in amixture.

It is still another object of the present invention to represent thepercentage of two known liquids in a mixture as a limited range voltagesignal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more fully apparent from the following detailed description ofthe preferred embodiment, the appended claims and in the accompanyingdrawings in which:

FIG. 1 is a functional block diagram showing the flexible fuel sensorand how it is interconnected to control circuitry and hardware relatedto the engine of a vehicle;

FIG. 2 is an overall flow chart of the methodology employed by theflexible fuel system described herein to detect the composition of thefuel sampled;

FIG. 3A is a schematic diagram of the circuitry of the flexible fuelsystem;

FIG. 3B is a continuation of the schematic diagram of the flexible fuelsystem shown in FIG. 3A;

FIG. 4 is top view of the housing in which the flexible fuel sensor ismounted;

FIG. 5 is a side view of the housing shown in FIG. 4;

FIG. 6 is a graph of the analog output signal from the flexible fuelsensor system vs. the percentage methanol sensed in the fuel; and

FIG. 7 is a graph of the percentage methanol sensed in the fuel vs. thefrequency output of the flexible fuel sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is known that the electrical properties of gasoline differconsiderably from those of methanol. One of those properties is ofparticular interest here, the dielectric constant of the two distinctliquids. More specifically, the dielectric constant of gasoline isapproximately 2.05, whereas it is 32.63 (@ 25° C.) for methanol. Becauseof the order of magnitude of difference between the dielectric constantof the two fuels, the dielectric constant is a desirable property tosense for determining the relative percentage of gasoline and methanolin a mixture thereof.

A method of detecting this property is by measuring the change indistributed capacitance of a capacitor with the fuel mixture as itsdielectric material. The measured capacitance is a direct indication ofthe composition of the mixture. Change in capacitance can be sensed as achange in the resonant frequency of a coil immersed in the mixture to bedescribed in greater detail below. The resonant frequency is a functionof the dielectric material and its value will change from its "air"value (i.e., where there is no dielectric material other than air) tosome value dependent upon the fuel mixture's combined dielectricconstant when the coil is immersed in the mixture. The subject inventionsenses the dielectric constant of the fuel mixture by using thedistributed capacitance of a free-standing "air core" (dielectricmaterial is air) inductor as a standard, comparing it with thedistributed capacitance generated when a fuel mixture acts as thedielectric material instead of air.

Referring to FIG. 1 the main elements of the subject invention areschematically shown. Fuel mixture sensing device 22 includes acylindrical coil 20 fabricated from magnetic wire which is insulatedwith a polyamidimide coating. The polyamidimide coating provides a longlife resistance to gasoline and alcohol fuels while providing thethinnest coating possible. Additionally, the coating is needed toeliminate shunt resistance (being generated between the loops of wirethat form the coil) and fluid ion mobility effects (i.e., theconductivity of the fluid itself), yet it must still be thin enough soas not to mask the dielectric constant of the mixture being measured.

Coil 20 is preferably constructed by winding a 20 gauge magnetic wire ina 0.375" diameter coil. 221/2 turns of wire produce a coil having thedesired characteristics when the coil is approximately 0.81" long. Coil20 has an inductance of approximately 2 μHenries.

Coil 20 is shown immersed in a fluid mixture 23 flowing through acylindrical casing or coil chamber 24. The fuel is pumped from anautomotive fuel tank (not shown) to a vehicle's engine (not shown). Itis to be appreciated that the coil 20 could be directly immersed in thefuel tank. Coil chamber 24 is provided with input and output portswhereby the mixture 23 can flow there through. In an internal combustionengine application, mixture 23 would consist of a fuel mixture ofgasoline and methanol and would flow through coil chamber 24 prior todelivery to the engine's fuel delivery system. It is contemplated thatsensor 22 could be installed anywhere within or between the fuel tankand the fuel rail of conventional motor vehicles.

The ends of sensing coil 20 are connected to an electrical circuit 30.Preferably, circuit 30 is interfaced with coil 20 immediately adjacentto coil chamber 24 to minimize the length of interconnections and anyresultant parasitic changes to the inductance or capacitance of thecircuit. A preferred configuration of the apparatus according to thesubject invention is illustrated in greater detail in reference to FIG.4.

Coil 20 is energized via circuit 30, and a capacitance is generatedbetween the loops of wire in coil 20 such that the loops of wire act asequivalent electrodes. The capacitance between two electrodes may bedefined for measurement purposes as the charge stored per unit potentialdifference between them. Capacitance is dependent on the area, spacing,and the character of the dielectric material between the electrodes asshown in the formula below:

    CAPACITANCE=(k)(A)/d

where "k" is the dielectric constant, "A" is the area circumscribed bythe loops of wire which form the equivalent electrodes, and "d" is thedistance between the electrodes. We are only concerned here with thedielectric constant of the mixture since the area and the spacing willremain constant.

Dielectric materials are electrical insulators in which an electricalfield can be sustained with a minimum dissipation of power. The fluidwith the higher dipole moment will have the higher dielectric constant.In a mixture of two known fluids, a proportional relationship existsbetween the percentage of each fluid in the mixture and its cumulativedielectric constant. This also means that capacitance will increase withan increase in percentage of the fluid possessing the higher dielectricconstant. It is this relationship which the subject invention exploits.

Coil 20 is electrically interfaced with circuit 30. In fact, coil 20 isused as the resonant element in an oscillator for generating a resonantfrequency which is related to the amount of one of the fluids (methanolin the preferred embodiment) present in the fuel mixture. The oscillatorportion of circuit 30 drives coil 20 until the resonant frequency of thecoil is determined. The oscillator is a tuned input type using coil 20and its distributed capacitance as a parallel resonant tank circuitwhich causes oscillation at the tank's resonant frequency. The change indielectric constant therefore, causes a change in the resonantfrequency. In this manner, resonant frequency decreases as capacitanceincreases according to the formula: ##EQU1## where "f" is frequency, "L"is the inductance of sensing coil 20, "C" is Capacitance, ε_(fl) is therelative dielectric constant of the fluid being measured, C_(o) is thebase distributed capacitance of the sensing coil, and k is a multiplierfor the effects of coil insulation and oscillator circuitry and includesall stray or parasitic impedances reflected across the inductorterminals.

This shows that C_(o) k represents the free air capacitance of the coiland oscillator and chamber (and associated housing, etc.; i.e. withoutthe effects of the fluid) and C_(o) k+C_(o) ε_(fl) represents the freeair capacitance and that of the coil in the fluid.

From the above relationships, it can be seen that as the percentage ofthe fluid with the higher dielectric constant increases, the resonantfrequency of coil 20 immersed within the mixture decreases. As will bedetailed hereinafter, the circuit 30 converts the frequency to a voltageand employs the voltage signal as an output.

Further reference to FIG. 1 illustrates the subject invention as appliedto an alcohol/gasoline mixture, and more preferably a methanol/gasolinefuel mixture for an internal combustion engine. The output signal fromcircuit 30 is sent to a microprocessor 80 which, in turn, selectivelyadjusts the spark timing and fuel ratio of engine (not shown) of avehicle (also not shown) for proper internal combustion.

However, it should be understood that the subject invention embodies thenovel concept of measuring changes in the resonant frequency of a tankcircuit comprising a coil with its inherent distributed capacitanceimmersed in a fluid mixture to determine the combined dielectricconstant of the mixture. In this manner, the specific value, number andinterconnection of components disclosed herein are presented toillustrate a preferred embodiment and are not to be construed to limitthe invention accordingly. Furthermore, design considerations affectingthe final configuration of coil 20 will likewise affect the componentvalues required for proper circuit operation.

FIG. 1 generally shows a functional block diagram of the subjectflexible fuel sensor system 18. All of the electrically powered blocksshown in FIG. 1 are powered from a DC input of between 6-24 Volts DCthrough a 5 V DC regulator 70.

The sensing coil 20 shown is a part of the oscillator circuit 30.Together, the sensing coil 20 and the rest of the oscillator circuit 30form the flexible fuel sensor 22 shown set off with dashed lines. Theflexible fuel sensor output signal is shown as 34.

The sensing coil 20 is in direct communication with the fluid to besensed. In the case of the preferred embodiment more specifically shownand described herein, the fluid to be sensed is a methanol fuel mixture(fuel) 23 shown in a reservoir or coil chamber 24. The sensing coil 20can be in a fuel tank, remote from the rest of the oscillator circuit 30or could be configured to sense a sample of fuel 23 and immerse the coil20 into coil chamber 24 remote from the fuel tank. In either event, thesensing coil 20 must be in direct communication with the fuel 23.

The flexible fuel sensor 22 can be set to oscillate at any frequency,however for purposes of the subject invention, the flexible fuel sensoroutput signal 34 oscillates in a range of frequencies between 13 MHz and21 MHz was chosen. This frequency range is a function of the size of thesensing coil 20 and of the characteristics of the fluid or fuel to besensed.

The flexible fuel sensor output signal 22 is fed into a divide by (÷)256 circuit shown at 40 in FIG. 1. This circuit is a function of theparticular electronics system used with the flexible fuel sensor 22 andmay not be needed with other circuitry. Similarly, the system 18 can bereproduced using combinations of discrete electronic components. Thedivide by 256 circuit 40 converts the flexible fuel sensor output signal34 to a frequency ranging between 50 KHz and 82 KHz. The flexible fuelsensor output signal 34 appears as divided flexible fuel sensor outputsignal 44 from the output of block 40. This divided flexible fuel outputsignal 44 is presented to microprocessor 80 for further analysis whichwill be explained below.

Oscillator temperature sensor 50 generates a oscillator temperaturesensor output signal 54 which is a function of the temperature of theoscillator 30 itself. In the case of the preferred embodiment shown anddescribed herein, the oscillator temperature sensor output signal 54 isa voltage. The oscillator temperature sensor output signal 54 is alsopresented to the microprocessor 80 for processing with the dividedflexible fuel sensor output signal 44. This also will be explained morefully below.

The preferred embodiment of the system 18 also includes a fueltemperature sensor 60 which, as in the case of the oscillatortemperature sensor 50, generates a voltage output. This is shown as fueltemperature sensor output signal 64 and is a function of the temperatureof the fuel 23 being sensed. It also is processed by the microprocessor80.

Microprocessor 80 compares the divided flexible fuel sensor signal 44along with the oscillator temperature sensor signal 54 and the fueltemperature sensor signal 64 to predetermined tables and/or curves ofempirical data to determine the methanol concentration of the sensedfuel 23. This information is contained in the microprocessor outputsignal 84.

A low pass filter 90 is also provided to convert the pulse widthmodulated microprocessor output signal 84 back to an analog voltageoutput 94. The information in either signal 84 or 94 can be used fordisplay purposes or sent to an engine computer (not shown) to changeappropriate sections of the operating characteristics of the engine inresponse and correlated to the concentration of methanol in the fuel 23.Output verification is also provided by feeding the signal 94 back tomicroprocessor 80 for comparison. This is a matter of design choice andmay not be necessary for a particular implementation of the circuitryand system described and claimed herein; in the example shown, it ismore of a development tool. Output protection can be provided forproduction versions of this circuit.

Referring now to FIG. 2, an overall flow chart is provided to show amethod of sensing the percent methanol in fuel utilizing the system 18shown in FIG. 1.

The method starts in block 100 and is called for by the system 18 eitheron a periodic basis or on an interrupt basis as needed.

Next, in blocks 110, 120, and 130 the system 18 calls for and senses thefrequency output of the sensor 22, the temperature of oscillator 30, andthe temperature of the fuel 23.

Then in blocks 140 and 150, the system 18 uses the information fromblocks 110, 120 and 130 to look up in the memory in microprocessor 80 orcalculate in microprocessor 80 the % methanol. It is a PWM modulatedoutput and is proportional to % methanol (10% -90%) duty cycle Theoutput of the system 18 will be a temperature compensated value thatcorresponds to the % methanol in the fuel sensed using the fuel sensor22.

The look up data in memory can be empirically derived by measuring thedielectric constant of various known concentrations of gasoline andmethanol. Real time computation can also be accomplished withmathematical models of the relationship between the dielectric constantmeasured and the concentration of methanol in the fluid (fuel) mixture.

Next the microprocessor 80 calls for the method to return to repeat themethod beginning with block 110 the next time it is required.

More specifically, the steps that the system 18 uses to convert thesensed voltages from sensors 22, 50 and 60 are further described in FIG.3 and illustrates a more detailed methodology to that described inconjunction with FIG. 2.

One way to sense the frequency of the flexible fuel sensor 22 is shownin blocks 112 and 114. In block 112, the microprocessor 80 counts thenumber of pulses on the output of oscillator 30 in the time period of τseconds. Then in block 114, the microprocessor 80 divides the number ofpulses sensed in block 112 by τ to get the frequency of the output fromflexible fuel sensor 22.

Blocks 122 and 124 in FIG. 3 correspond to block 120 in FIG. 2. In orderto sense the temperature of oscillator 30 in block 120, themicroprocessor 80 will read values from the on board temperature sensor50. The output from the particular sensor 50 used in this application iscalibrated in the range from 0-5 volts. Then in block 124, the voltageoutput from temperature sensor 50 will be converted to a temperature foruse in computation or memory table look up steps by microprocessor 80.The conversion will be in the range of (-40° C. to +120° C). Thisconversion is retained in the memory of microprocessor 80.

Similar sense and conversion steps are performed in blocks 132 and 134in FIG. 3 which correspond with the sensing of the fuel temperature inblock 130 in FIG. 2. More specifically, in block 132 the microprocessor80 will read the voltage output from the fuel temperature 60 which forthe particular sensor 50 used is calibrated from 0 to 5 volts. Theconversion to a temperature value that can be used by microprocessor 80for compensatory computation/look up purposes later in the methodology,is done in block 134 in the range of (-40° C. to +120° C).

Referring to FIG. 3A and to FIG. 3B, presented is a schematic diagram ofthe circuitry of the flexible fuel sensor system 18 as shown in blockform in FIG. 1.

All of the electrically powered components shown are powered from a DCinput of between 6 and 24 volts D.C. through a 5 volt DC Regulator 70.The heart of this regulator is a regulator chip U5. Several commerciallyavailable components are available to accomplish this function. Thisparticular chip in the preferred embodiment employs a capacitive networkmade up of feed-through filter capacitors C17a and C17b to limit radiofrequency interference, input filter capacitor C11 for smoothing rippleof the voltage signal supplied by VIN (6-24 volts) and output filtercapacitor C12 for stability and ripple rejection on the output ofregulator chip U5 . The VIN represents voltage available from the supplysystem in an automotive electrical power system (not shown). Theresultant output from the regulator 70 is a 5 Volt D.C. which islabelled Vcc.

The sensing coil 20 is a part of the oscillator circuit 30 as previouslyshown in FIG. 1. Together, the sensing coil 20 and the rest of theoscillator circuit 30 form the flexible fuel sensor 22.

Upper bias resistor R1 and Lower bias resistor R2, along with couplingcapacitor C6 and sensing coil 20 form the input network to the rest ofthe oscillator circuit 30. Invertor/Amplifiers U1a, U1b and U1c form theheart of the oscillator operation along with feedback resistor R20across U1a, feedback capacitor C1 across the output of U1b and the inputto U1a, and bypass filter capacitor C2.

In operation, the sensing coil 20 is in direct communication with thefuel 23 (see FIG. 1) which in the preferred embodiment is the fluid tobe sensed. The sensor 22 can be set to oscillate at any frequency, butfor purposes of the subject invention (to sense the concentration ofmethanol in the fuel), the sensor produces an output 34 (see FIG. 1)with a frequency range between 13 MHz and 21 MHz.

The frequency of oscillation is determined by the coil L (shown as 20 inthe figures) and its distributed capacitance C (shown as 21 in thefigures). The oscillator 30 can be described as a tuned input oscillatorwhere L and C form a parallel resonant tank circuit.

From feedback theory it is known that a requirement for oscillation is aloop gain greater than or equal to unity along with zero or 360 degreesof phase shift around the loop. The gain for this oscillator is providedby two unbuffered high speed CMOS inverters (U1a and U1b). Using a CMOSdevice in a standard hex package promotes thermal stability due to thecomplementary nature of the device. This reduces the propagation delayand input capacitance of the devices for better operation at thefrequencies of interest.

The capacitor C1 serves as a feedback element in the circuit, providingpositive feedback to promote regeneration and oscillation at theresonant frequency. C2 provides a ground path for the AC voltagedeveloped across the coil while still allowing a DC bias set by R1 andR2 to exist at the input of U1a.

Resistor R20 provides some negative feedback on the first amplifierstage (U1a) in order to reduce its gain and flatten its transfercharacteristic. This ensures that the oscillator will start under allconditions and helps to eliminate component variability.

C3 is provided to AC couple the enclosure around the sensing coil 20(and related circuitry) to the circuit ground. This has a two-foldeffect. It decreases the effective distributed capacitance kC_(o) of thesensing coil 20 and allows more of a frequency shift to occur whenchanging the dielectric fluid in the chamber, and it provides shieldingto prevent RF radiation from the circuit by shunting the eddy currentsgenerated in the enclosure to circuit ground. It is not DC coupled sothat varying ground potentials on the enclosure will not causevariations in the circuit's ground potential.

The output from U1b is then fed through U1c to isolate and buffer theoscillator output from any loads placed on it. The high frequency outputis obtained at the output of U1c.

It is also important to use standard practice and place a bypasscapacitor C4 across the power supply to U1 to bypass any high frequencyswitching currents and prevent them from infiltrating the supply lines.

The capacitance of the tank circuit is determined by several factorswhich can be accounted for by the following formula:

    c=c.sub.o (k+ε.sub.fl);

where:

c is the distributed capacitance of the tank circuit;

c_(o) is the distributed capacitance of the coil free standing in avacuum with no insulation;

k is a constant which includes the effects of the insulation on the wirethe effects of the housing and the input impedance of the amplifier andbias network;

ε_(fl) is the dielectric constant of the fluid being sensed;

kC_(o) is the effective distributed capacitance of the tank circuit;

ε_(fl) C_(o) is the additional capacitance determined by the dielectricconstant of the fluid.

The output signal 34 is fed into a divide by 256 circuit 40. Except fora bypass filter capacitor C3 and some connections between various pinsas shown in FIG. 3A, the circuit comprises a binary ripple counter U2which is commercially available as a IC number 74HC393 (any 8 bitcounter will work to divide by 256). As shown in FIG. 1, the output ofthe divide by 256 circuit 40 is the divided flexible fuel sensor outputsignal 44. The divide by 256 circuit 40 converts the flexible fuelsensor output signal 34 to a frequency ranging between 50 KHz and 82KHz. The signal 44 is presented to microprocessor 80 along with thesignals from the oscillator temperature sensor 50 and the fueltemperature sensor 60.

Oscillator temperature sensor 50 generates an output signal 54, as shownin FIG. 1, which is a function of the temperature of the oscillator 30itself. The output 54 is in the form of a voltage. The sensor 50comprises bias thermistor RT2 and voltage divider resistor R11. Thechoice of this temperature sensor is directed by parameters that are notnecessarily unique to the flexible fuel sensor system described andclaimed herein; many other types of temperature sensors would work.

Fuel temperature sensor 60 generates an output signal 64, as shown inFIG. 1, which is a function of the temperature of the fuel 23 (in coilchamber 24 or in the fuel system (not shown)). The output 64 is also inthe form of a voltage. The sensor 60 comprises divider resistor R7 whichworks with the thermistor RT1. The choice of this temperature sensor isdirected by parameters that are not necessarily unique to the flexiblefuel sensor system described and claimed herein; many other types oftemperature sensors would work.

Microprocessor 80 compares the divided flexible fuel sensor signal 44along with the oscillator temperature sensor signal 54 and the fueltemperature sensor signal 64 to predetermined tables and/or curves ofempirical data to determine the methanol concentration of the sensedfuel 23. This comparison information is contained in the microprocessoroutput signal 84.

The heart of the microprocessor 80 circuit is microprocessor chip U3which is a commercially available chip, such as a MC68HC05B4,MC68HC05B6, MC68HC805B6, or MC68HC705B5 with the interconnections asshown. The chip U3 contains a memory (not shown) in which is stored thepredetermined relationships which allow the comparison of the sensedfuel to be compared with known characteristics of the concentrations ofmethanol in the fuel. This process is described in detail with respectto the method shown in FIG. 2.

The chip U3 is shown in FIG. 3B along with the external components asnecessary: pull up resistors R3 and R4, filter capacitor C4, resetcapacitor C5, clock shunt resistor R5 and clock capacitors C7 and C8,and crystal resonator Y1.

A low pass filter 90 converts the pulse width modulated microprocessoroutput signal 84 back to an analog voltage output 94, shown in FIG. 1.The information in either signal 84 or 94 can be used for displaypurposes or sent to an engine computer (not shown) to change appropriatesections of the operating characteristics of the engine in response andcorrelated to the concentration of methanol in the fuel 23. Outputverification is also provided by feeding the signal 94 back tomicroprocessor 80 for comparison. This is a matter of design choice andmay not be necessary for a particular implementation of the circuitryand system described and claimed herein; in the example shown, it ismore of a development tool. Output protection can be provided forproduction versions of this circuit.

More specifically, low pass filter 90 comprises a voltage dividercomprising resistors R6 and R13, input resistor R17, resistor R18,feedback capacitor C9 and capacitor C10 along with amplifier U4a, andfeedback capacitor C13. In addition, resistor R16, feedback resistorR15, gain control resistor R14, and filter capacitor C14 are providedalong with output clamping diodes D2 and D3 to complete the low passfilter design. This is a 2nd order Butterworth low pass filter ofconventional design and others would also work.

Capacitor C16 is provided to AC couple the enclosure to the sensorground and output filter capacitor C17c is provided to limit radiofrequency feed through interference on the output.

Referring now to FIGS. 4 and 5, shown is housing 200 which, in cavity250, encloses a circuit board (not shown) carrying the components of thesystem 18 described herein. The housing is provided with flanges 210 foruse with mounting hardware to secure it to a suitable location on thevehicle. Also provided is a connector means 220 to allow for theelectrical interconnection between the circuit board carrying thesubject system circuitry and the rest of the vehicle's engine controlhardware. Fuel inlet/outlets 230 are also provided to communicate withfuel line tubing (not shown) which will transport fuel 23 to and fromthe inlet/outlets 230 which in turn allow the sensing coil 22 (alsoshown as coil bobbin 240 in FIG. 4) to interact with the fuel 23.

Referring now to FIG. 6 shown is a graph of the analog output signalfrom the flexible fuel sensor system vs. the percentage methanol sensedin the fuel. FIG. 7 shows a graph of the percentage methanol sensed inthe fuel vs. the frequency output of the flexible fuel sensor. These twocurves are merely representative of the approximate relationship betweenthe voltage and frequency outputs as related to the percentage methanolcontained in the fuel 23. The shapes of the curves will change slightlydepending on the scale used and the choice of the hardware used, theamount of fuel present to sense, the temperatures involved, etc.

It should be understood that while this invention has been discussed inconnection with one particular example, those skilled in the art willappreciate that other modifications can be made without departing fromthe spirit of this invention after studying the specification, drawingsand the following claims.

We claim:
 1. A method of determining the respective percentages of firstand second liquids in a mixture thereof, comprising the stepsof:immersing an inductive element in said mixture such that said mixtureat least partially determines a value of the distributed capacitanceexhibited by the inductive element; energizing said inductive element togenerate a signal having a frequency determined by said inductiveelement and by its distributed capacitance; and detecting the frequencyof said signal.
 2. The method of claim 1 wherein said mixture at leastpartially determines the value of said distributed capacitance of saidinductive element in accordance with respective dielectric constants ofsaid first and second liquids.
 3. The method of claim 2 wherein saidinductive element comprises a coil having a known inductance such thatsaid distributed capacitance of said coil is proportional to a combineddielectric constant of said mixture which is indicative of therespective percentages of said first and second liquid.
 4. The method ofclaim 3 wherein the frequency of said signal is the resonant frequencyof said coil such that a change in the dielectric constant of saidmixture produces a proportional change in the distributed capacitancewhich, in turn, produces a change in the resonant frequency of saidsignal substantially according to the following equation: ##EQU2## where"f" is the frequency of said signal, "L" is the known inductance of saidcoil, and "C" includes at least the value of the distributed capacitanceof said coil.
 5. A method of determining the percentage of alcohol in agasoline/alcohol fuel mixture for selectively controlling combustionparameters of an internal combustion engine, comprising the stepsof:immersing an inductive element in said fuel mixture such that saidfuel mixture at least partially determines a value of the distributedcapacitance exhibited by said inductive element in accordance with acombined dielectric constant of said fuel mixture which is indicative ofthe percentage of the respective dielectric constants of alcohol andgasoline in said mixture; energizing said inductive element to generatea signal having a frequency determined by said inductive element and byits distributed capacitance; and detecting the frequency of said signalbeing indicative of said percentage of alcohol within said mixture. 6.The method of claim 5 wherein the alcohol in said fuel mixture ismethanol.
 7. The method of claim 5 wherein the step of immersing aninductive element in said fuel mixture further comprises installing afuel mixture sensing device in fluid communication with said fuelmixture between a fuel tank and said engine, said inductive elementassociated with an internal chamber provided in said sensing devicethrough which said fuel mixture flows.
 8. The method of claim 7 whereinsaid inductive element is a coil winding coupled to a source ofelectrical energy, said coil acting as a resonant element in a parallelresonant tank circuit such that the frequency of said signal is theresonant frequency of said tank circuit, said resonant frequency beinginversely proportional related to the distributed capacitance of saidcoil substantially according to the following equation; ##EQU3## where"f" is the frequency of said signal, "L" is the inductance of said coil,and "C" includes at least the value of the distributed capacitance ofsaid coil.
 9. The method of claim 5 further comprising the step ofconverting said frequency signal to a signal indicative of thepercentage of methanol in said fuel mixture.
 10. The method of claim 9wherein said frequency signal is converted into an output signal withina specified range corresponding to a range of the percentage of methanolpresent in said fuel mixture.
 11. The method of claim 10 furthercomprising the step of inputting said output signal to an enginecontroller for selectively controlling said combustion parameters. 12.The method of claim 11 wherein said combustion parameters include sparktiming and air/fuel ratio.
 13. A method of determining the percentage ofalcohol in a gasoline/alcohol fuel mixture for controlling combustionparameters of an internal combustion engine of the type used in motorvehicles, comprising the steps of:installing a fuel mixture sensingdevice between a fuel tank and said engine of said vehicle such thatsaid sensing device is in fluid communication with said fuel mixture,said sensing device having an inductive coil winding coupled to a sourceof electrical energy with said inductive coil winding immersed in saidfuel mixture such that said fuel mixture at least partially determines avalue of the distributed capacitance exhibited by said coil; energizingsaid coil winding to generate a signal having a frequency determined bythe distributed capacitance of said coil winding in accordance with therespective dielectric constants of the alcohol and gasoline; detectingthe frequency of said signal which is indicative of the percentage ofalcohol and gasoline within said fuel mixture; and converting saidfrequency signal into an output signal within a specified rangecorresponding to the percentage of alcohol present in said fuel mixture;and inputting said signal to an engine controller for selectivelycontrolling said combustion parameters.
 14. The method of claim 13wherein said fuel mixture sensing device comprises a housing defining achamber within which said coil winding is confined and a circuitincluding oscillator means for oscillating said coil winding at itsresonant frequency, said circuit further comprising frequency detectionmeans for detecting said resonant frequency signal and signal conversionmeans for converting said frequency signal into said output signal. 15.An apparatus for determining the respective percentages of first andsecond liquids in a mixture disposed within a chamber, comprising:aninductive element immersed in said mixture such that the mixture atleast partially determines a value of distributed capacitance exhibitedby said element; a source of electrical energy coupled to said inductiveelement for energizing said element so as to generate a signal having afrequency determined by said inductive element and its distributedcapacitance; and frequency detection means coupled for receipt of saidsignal generated by said inductive element in response to energizationthereof.
 16. The apparatus of claim 15 wherein the frequency detectionmeans includes convertor means for converting said frequency of saidsignal into a signal indicative of said percentages.
 17. The apparatusof claim 15 wherein said mixture at least partially determines the valueof said distributed capacitance in accordance with respective dielectricconstants of said first and second liquids.
 18. The apparatus of claim17 wherein said inductive element is a coil acting as a resonant elementin an oscillator, said oscillator being of the tuned input type suchthat said coil and its distributed capacitance define a parallelresonant tank circuit which generates an oscillating signal at aresonant frequency of the tank circuit, said resonant frequency beinginversely proportional related to said distributed capacitance of saidcoil substantially in accordance with the following equation: ##EQU4##where "f" is the frequency of said signal, "L" is the inductance of saidcoil, and "C" at least includes the value of the distributed capacitanceof said coil.
 19. The apparatus of claim 16 wherein said convertor meanscomprises a frequency to voltage convertor circuit.
 20. A fuel mixturesensing apparatus for determining the percentage of alcohol in agasoline/alcohol fuel mixture for use in a motor vehicle having aninternal combustion engine, comprising:a housing having an inlet and anoutlet and defining an internal chamber therebetween in fluidcommunication with said fuel mixture; an inductive coil winding confinedwithin said chamber and immersed in said fuel mixture such that themixture at least partially determines the value of distributedcapacitance exhibited by said coil winding; a source of energy coupledto said inductive coil winding for energizing said coil winding togenerate a signal having a frequency determined by the distributedcapacitance of said coil winding; and frequency detection means coupledfor receipt of said signal generated by said coil winding in response toenergization thereof, and convertor means for converting said frequencysignal into a voltage signal indicative of the percentage of alcohol insaid fuel mixture.
 21. The fuel mixture sensing apparatus of claim 20wherein said mixture at least partially determines the value of saiddistributed capacitance in accordance with respective dielectricconstants of said alcohol and gasoline in said fuel mixture.
 22. Thefuel mixture sensing apparatus of claim 21 wherein said coil winding andits distributed capacitance define a parallel resonant tank circuit suchthat the frequency of said signal is the resonant frequency of said tankcircuit, said resonant frequency being inversely proportional related tothe distributed capacitance of said coil winding substantially accordingto the following equation: ##EQU5## where "f" is the frequency of saidsignal, "L" is the inductance of said coil, and "C" includes at leastthe value of the distributed capacitance of said coil.
 23. The fuelmixture sensing apparatus of claim 20 further comprising electricalconnection means for inputting said voltage signal to an enginecontroller for selectively controlling combustion parameters.